This invention refers to a spray gun, of the type used in the industrial thermal spray area for obtaining coatings, especially in detonation spray technologies.
The object of the invention is to achieve a new detonation gun with greater productivity than existing ones, maintaining stable and continued optimum spray conditions in each firing cycle. In relation to previous detonation devices, this gun allows the firing frequency to be increased, together with the amount of powder and feeder gases and in consequence, the amount of coating powder deposited per unit of time, maintaining optimum levels of quality that are characteristic of coating produced by detonation technologies.
For this purpose, a new gas feeding system is proposed, in a new explosion chamber, that permits the gun""s operating frequency to be increased, making it possible to maintain the optimized characteristics of each explosion stable and constant, even at high frequencies and a new system for feeding products in the barrel that allows the distributed injection of products to any point within the barrel achieving an increase of the amount of powder injected into the barrel and reducing the limitations associated with obstruction of feeder ducts, together with great operating versatility by being able to select the injection point.
The barrel feeding system, in addition to the coating powder, it is also useful to introduce other products that can condition the thermal spray process, in this way permitting great flexibility when modifying the operating parameters, by being able to modify the characteristics of the generated explosions and to improve and optimize the coatings obtained in this way.
It is also an object of the invention to achieve better performance from the gun, based on thermally isolating the gases produced in the explosive process with respect to the cooled barrel wall, in order to obtain better use of the energy that is carried by these gases, with the resulting increase in the gun""s performance and its efficiency.
Current detonation spray technologies are mainly used for the application of coatings to parts that are subject to severe conditions of wear, heat or corrosion, and which are fundamentally based on the use of the thermal and kinetic energy produced by the explosion of a gaseous mixture to deposit a coating material powder on these parts.
The coating materials that are usually employed in detonation spray processes include metallic powder, metal-ceramics and ceramics etc, and are applied to improve the resistance to wear, erosion, corrosion and as thermal insulators or as electrical insulators or conductors, among other applications as given in the literature.
Detonation spray is performed with spray guns that basically consist of a tubular explosion chamber with one end closed and the other open, to which a barrel, also tubular, is connected. The explosive gases are injected inside the explosion chamber and ignition of the gas mixture is produced by means of a spark plug, which provokes an explosion and in consequence, a shock or pressure wave that reaches supersonic speeds during its propagation inside the barrel until it leaves the open end.
The coating material powders are usually injected inside the barrel in contact with the explosive mixture so that they are dragged along by the propagating shock wave and by the set of gaseous products from the explosion, which are expulsed at the end of the barrel, and deposited on a substrate or part that has been placed in front of the barrel. This impact of the coating powders on the substrate produces a high density coating with elevated levels of internal cohesion and adherence to the substrate. This process is repeated in a cyclic manner until the part is suitably coated.
In traditional detonation spray equipment, the gases used in the generation of the explosive process are mixed in a separate chamber prior to the explosion chamber, which is then fed by a homogeneous mixture of gases in each explosive cycle. Traditionally, this pre-mixing chamber is isolated from the explosion chamber during the explosive phase for safety reasons, through the use of valves in one or more gas lines, with and without the introduction of an inert gas between two consecutive explosions.
In other, more advanced types of detonation equipment, presented by the applicant in PCT US96/20160, this isolation between the pre-mix and explosion chambers is achieved by using dynamic valves, which means they do not have any moving parts, which overcomes the inherent disadvantages of the previously-mentioned mechanical systems. However, these devices continue to employ a pre-mixing chamber in order to homogenize the gas composition that feeds the explosion chamber.
Recently, the same applicant developed a type of detonation spray equipment, described in PCT ES97/000223, with a gas injection system that does not employ mechanical valves or systems to shut off the gas supply, and, in addition, allows the gases feeding to be fed directly and separately to the explosion chamber through a series of independent passageways, where each passageway is made up of an expansion chamber and a large number of distributor ducts with reduced cross section and/or long length. This results in a system without any moving mechanical parts and/or pre-mixing chamber. In this device, the expansion chamber for each passageway is in direct communication with the corresponding supply line, while the distributor ducts are suitably arranged so that multiple gas injection points open out on the internal surface of the explosion chamber, producing a continuous and separate feeding at multiple points, which guarantees that the combustible mixture is produced directly and in a homogeneous manner, throughout the entire explosion chamber prior to each ignition and with sufficient flow to fill the chamber in each detonation cycle.
In turn, in the application PCT ES98/00015, also of the same applicant, a powder injection system is described for a detonation spray gun consisting of a dosing chamber directly fed by a conventional type continuous powder feeder that communicates with the barrel by means of a direct duct. In this way, the pressure generated by the explosion and which advances along the barrel, passes through the communication duct and undergoes a brusque expansion on reaching the dosing chamber, which interrupts the powder feeding from the continuous feeder and produces complete fluidization of the powder in the dosing chamber. The fluidized powder is carried by the suction towards the barrel, where the pressure wave generated in a new explosive cycle drags it out and deposits it on the surface to be coated.
The detonation guns of the described type produce coatings of excellent quality, but they have a limitation in so far as the amount of powder that can be deposited per unit of time. This is due to the fact that, for a detonation gun of a determined size, the optimum amount of powder that can be processed during each explosion is limited by the existence of a maximum volume of optimized gaseous mixture that may be processed in each explosion and capable of generating proper characteristics of the actual explosive process itself. An increase in the gaseous volumes involved in each explosion on this maximum volume of optimized mixture is not directly translated into an improvement of the explosive process of each cycle, so that an increase in the amount of powder deposited per unit of time should not be obtained so much because of an increase in the powder processed in each explosion, but as a consequence of the increasing in the firing frequency, guaranteeing optimum explosive characteristics of each cycle in all cases.
On the other hand, the repetition of the explosive cycle at high frequencies and generating explosions with characteristics equivalent to those obtained at lower frequencies also requires higher gas flows in order to guarantee constant gas volumes involved in each explosion. The application of these increments in the gas flows and in the firing frequencies in the previously described equipment produces an increase in the gun""s power rating and an increase in the gas supply pressure with an acceleration in the injection and gas mixture processes inside the explosion chamber which causes great difficulty in the maintenance of the actual cyclic detonation process itself, leading to continuous combustion processes and making the spray process impossible with that equipment. In particular, an increase in the gun""s power rating and consequently in the gas injection system temperature makes more difficult the cooling of the gases produced in an explosive cycle and which, returning through the injection system ducts allows the cyclic interruption of the supply of oxidizer and fuel to the chamber.
In the equipment described in PCT ES97/00223, the gases, on their return to the explosion chamber, act as an insulating barrier between the gases produced in the prior explosive cycle and the new gas mixture formed in the explosion chamber, preventing self-ignition. However, the operation of this mechanism at high frequencies is made difficult by an increase in the temperature of the explosion chamber, a reduction in the volume of the return gases that acts as an insulating barrier and their rapid return to the explosion chamber, as a result of the greater pressure in the feed lines. In the previously described detonation devices, this leads to the self-ignition of the combustible mixture and the formation of a continuous combustion process.
In currently existing detonation guns as described in this section, there is an additional limitation that derives from the types of powder feeders used since they cannot guarantee the correct fluidity of the powder at high supply speeds. In this sense, it can be seen that current designs are subject to major problems of obstruction and wall deposits on the feeding ducts above a certain amount of injected powder, and this makes continuous and stable operation very difficult. This is mainly due to the geometric aspects of the powder injection devices and/or thermal aspects in relation to the explosive process. In the injection device described in PCT ES98/00015 from the same applicant, the powder is introduced into the barrel through a single orifice, then carried along by the hot gases generated in the explosive cycle. Any increases in the amount of powder, gases and in the operation frequency in order to increase the productivity of the spray process, will soon come up against a limit in the feeding devices, such as that previously stated, since as a consequence of the accumulation of material in a localized area and in the increase of temperature of the gases that interact with the powder in the injector, obstruction and deposit problems as stated before are produced.
On the other hand, there are spray technologies, known as HVOF, that do not produce cyclic explosions, but a continuous combustion that it used in the formation of a supersonic flow of hot gases that are actually employed in the thermal spray process, requiring, in this case, very high gas flow rates for maintaining this required supersonic flow rate for obtaining coatings with a good technical quality.
Due to the continuous nature of the HVOF processes, the more advanced designs of HVOF guns have a powder processing capacity per unit of time that exceeds that achieved with traditional detonation spray systems, although they still have similar problems in the injection of powder, obstruction and deposits inside the spray nozzles.
However, the lower thermodynamic efficiency of the continuous combustion processes against the explosive processes (pulsed or cyclic combustion) leads to the fact that the amounts of gases and power required to deposit the same amount of powder is greater in the HVOF systems, which results in lower performance in resource use and in the introduction of additional operational problems as a consequence of the high working powers employed in the HVOF systems with high processing capability.
It would be therefore, desirable to have a spray gun that employs a pulsed explosive process, with high thermodynamic efficiency in the use of gases and precursor materials, allowing a significant increase in the amount of powder processed per unit of time, and maintaining the typical characteristics of the coating produced by the detonation technologies.
The detonation spray gun of the invention, allows the working at higher frequencies than those employed in currently existing devices with a large volume of powder feeding, achieving greater deposit rates, even when compared with those obtained with current HVOF continuous combustion equipment, but maintaining the higher thermodynamic efficiency of the explosive processes in the use of the gases and precursors, resulting in greater productivity.
The current detonation spray system is based on the generation of explosive gaseous mixtures of different compositions in different zones of the chamber zone, which is due to a specific design of the gas injectors and the explosion chamber, employing dynamic valves and direct, separate injection for fuel and oxidizer, without pre-mixing of both prior to the explosion chamber itself.
First, in order to enable the gun to operate at high frequencies with high gas volumes per explosion, it has been planned for the gas feeding to the explosion chamber to be produced via several points, spatially distributed throughout the explosion chamber, so that gaseous mixtures are generated with locally varying compositions in the various zones inside this chamber, allowing higher energy explosions to be generated at higher frequencies and maintaining stable cyclic operation.
Inside the explosion chamber, just before the orifices employed for oxidizer feeding, there is a protuberance or internal perimeter rib that determines a narrowing of the internal diameter of the explosion chamber, defining an annular volume which is fed exclusively with fuel through multiple distributors arranged in the rearmost zone of the explosion chamber. This constrained volume favors thermal interchange of the gases produced in the explosion with the cooled chamber wall and also allows an increase in the gas volume that acts as an insulating barrier between the gases involved in two consecutive explosive cycles, and in this way simplifies the maintenance of the pulsed process under the circumstance imposed by the high gas flow rates and high frequency that are the object of this patent.
In accordance with this operating scheme, after each ignition of the spark plug, the propagation of a shock and temperature wave generated by the explosive process, returns to the said constrained annular volume producing the combustion and decomposition of the fuel present in this volume, together with an overpressure that produces an interruption of the fuel feeding supply and even the penetration of the products of combustion via the distribution ducts. The high gas flow rates required in order to work at high frequencies cause this latter factor to be reduced so that new fuel is able to rapidly penetrate the explosion chamber via the distribution ducts, however, this effect is compensated by the presence of this constrained annular volume in the explosion chamber, the content of which in combustion products generates a sufficient amount of gas to act as an insulating barrier between the hot gases originated in the previous explosion and the new gases supplied to the explosion chamber.
The feeding of oxidizer begins in the zones closed to the ignition point (spark plug) to generate a local mixture poor in oxygen, with an injection in this zone of a maximum of 25% of the total volume supplied in each cycle, together with the local injection of the totality of fuel supplied to the explosion chamber.
The rest of the oxidizer is introduced into the explosion chamber in more advanced positions, closer to the tubular barrel, so that the combustion front that is produced at each spark plug ignition meets up with mixtures that are richer in oxidizer as it progresses along the explosion chamber, increasing its speed and energy, producing very energetic explosions that are suitable for the production of high quality coatings.
In this way, it is possible to produce, within the same chamber volume, and for the same explosive cycle, zones of greater and lesser energy. In particular, the new design of explosion chamber and the gas injection system favors the supply of energy to the zone closer to the oxidizer injection, and at the same time reduces the energy of the explosion in the rearmost zone of the explosion chamber, thus increasing the efficiency of the injection system in cooling the gases that accompany the retreating pressure wave and favoring the continuity of the cyclic detonation process at higher frequencies than with the previous devices.
According to a preferable construction, the oxidizer injector is concentrically and internally arranged in the explosion chamber, and has a prolongation at one end that extends practically to the gun""s barrel, this prolongation incorporating a series of orifices obliquely arranged with respect to the gun""s barrel, for the injection of oxidizer in this advanced location in the explosion chamber.
A second characteristic of the gun object of this invention, refers to the incorporation of a system for feeding products at any point of the barrel, a system that when it is used for the injection of coating powder permits an increasing of the amount of powder feed to the gun per unit of time, and therefore the amount of powder deposited on the substrate per unit of time, increasing also the gun""s productivity.
For this reason, the barrel comprises an annular chamber at an intermediate point of the barrel, assisted by one or more material feeding inlets, so that the product introduced through them reaches the inside of the barrel with an annular distribution achieving a good mixture with the gases that are present in the barrel and avoiding the formation of high concentrations of material in specific zones, just as occurs with traditional injectors consisting of radial orifices.
The employment of this type of feeding ducts for the injection of the coating powder permits good distribution of the powder because, instead on entering the barrel through a single point, it does so through the annular chamber and consequently in a more homogeneously distributed manner, reducing the volumetric density of powder injected per unit of area, reducing the problems of blockages, but, in addition, allowing a larger amount of powder to be introduced into the gun.
In accordance with another characteristic of the invention, it has been planned for the mentioned annular chamber to take the form of a flange that divides the chamber in two segments, to allow the flange to be dismounted for injection duct maintenance and the front part of the barrel corresponding to the exit mouth in order to replace it with one having different characteristics, so that the same gun may have several configurations, including various lengths that allows coatings with different materials that require greater or less thermal and/or kinetic energy and hence a longer or shorter barrel.
In a similar fashion, it is also possible to connect segments of barrel having different diameters according to the type of coating powder used or the special characteristics of the current process or application.
It has also been planned for the flange that incorporates the annular injector to be coupled to the gun by means of a device that allows the separation between the flange and the barrel to be varied to established and entrance of external air between the two parts, and even to make one part independent from the other, so that on certain occasions the performance and results of the gun can be improved.
In accordance with another of the invention""s characteristics, it has also been planned that the flange comprises a second annular chamber, with its corresponding inlets for feeding material and which opens to the inside of the barrel and chamber to allow the injection of a product of the same or different characteristics of the one introduced via the main chamber. Specifically, it is possible to introduce powders of different types or to distribute the powder feeding along the length of the barrel, which will permit to obtain a greater versatility in the composition of the coatings obtained.
It is also possible to use the mentioned annular feeding system for the injection of active gases, in such a way that it would be possible to locally modify the nature of the mixture conditioning the explosive process, so, for example, these active gases may modify the energetic characteristics of the actual spraying process itself, modifying the temperatures and speeds applied to the sprayed particles or they can also provide a thermochemical environment that conditions the reactive interaction between these gases and the particles to be deposited, or even produce the synthesis of the materials deposited during the spray process.
Of course, the described annular injector may be single, double or multiple, comprising one or several product feeding inlets and one or more injectors of this type can be distributed along the barrel.
Therefore, by means of the proposed feeding system, it is possible to voluntarily modify the gun""s working conditions, since it is possible to inject all types of products that may modify, both the spray process conditions and the coating composition, and this injection may be made at any point of the barrel and so, as already mentioned, the dimensions of the barrel may be rapidly and simply changed, achieving an enormous flexibility in the gun""s operation and consequently in its capability of processing a wide range of material.
It is also possible to use the described annular injector for the introduction of an inert gas to reduce the transfer of heat between the gases produced in the explosion and the cooled wall of the barrel, thus making use of these gases to best advantage.
In accordance with this structure, the gases produced in the explosion progress along the central zone of the barrel in its output sector, while the gases injected by means of the cited annular chamber flow in contact with the barrel wall, forming a kind of moving cylindrical film that reduces the heat losses of the gases produced in the explosion through contact with the cooled tube that forms the barrel and which determines greater performance from the gun.
In addition, the film of surrounding gases form at the mouth of the barrel what could be called a virtual barrel, that axially lengthens the size of the actual barrel itself, reducing and delaying the mixture of the explosive process products with the gases in the environment, which leads to the fact that with a shorter, lighter barrel, the powder particles are better melted and this produces a coating with better properties.
When using easily oxidized powders, it is possible to carry out the injection with an inert gas, so that the powder is protected from the environmental air by being surrounded by this gas and consequently, the quality of the produced layer or coating is improved.